python中的dup,dup2,tmpfile和stdout

Question

这是从一个跟进的问题在这里.

---

我想去哪里

我希望能够暂时将stdout重定向到临时文件,而python仍然可以打印到stdout.这将涉及以下步骤:

1. 创建stdout(new)的副本
2. 创建临时文件(tmp)
3. 将stdout重定向到 tmp
4. 告诉python使用newstdout
5. 重定向tmp到"真正的"标准输出
6. 告诉python再次使用"真正的"标准输出
7. 阅读和关闭 tmp

履行

我尝试通过以下方式实现上述方法:

import os
import subprocess
import sys

#A function that calls an external process to print to stdout as well as
#a python print to pythons stdout.
def Func(s, p = False):
    subprocess.call('echo "{0}"'.format(s), shell = True)
    if p:
        print "print"

sil = list() # <-- Some list to store the content of the temp files

print "0.1" # Some testing of the
Func("0.2") # functionality

new = os.dup(1)    # Create a copy of stdout (new)
tmp = os.tmpfile() # Create a temp file (tmp)

os.dup2(tmp.fileno(), 1)            # Redirect stdout into tmp
sys.stdout = os.fdopen(new, 'w', 0) # Tell python to use new as stdout

Func("0.3", True) # <--- This should print "0.3" to the temp file and "print" to stdout

os.dup2(new, 1)                   # Redirect tmp into "real" stdout
sys.stdout = os.fdopen(1, 'w', 0) # Tell python to use "real" stdout again

# Read and close tmp
tmp.flush()
tmp.seek(0, os.SEEK_SET)
sil.append(tmp.read())
tmp.close()

我想在这里稍作休息来总结一下.
到此为止的控制台输出应为:

0.1
0.2
print

虽然sil应该是这样的:['0.3\n'].所以一切都像一个魅力,直到这里.但是,如果我再次重做上面的脚本:

print "1.1" # Some testing of the
Func("1.2") # functionality

new = os.dup(1)    # Create a copy of stdout (new)
tmp = os.tmpfile() # Create a temp file (tmp)

os.dup2(tmp.fileno(), 1)            # Redirect stdout into tmp
sys.stdout = os.fdopen(new, 'w', 0) # Tell python to use new as stdout

# This should print "0.3" to the temp file and "print" to stdout and is the crucial point!
Func("1.3", True) 

os.dup2(new, 1)                   # Redirect tmp into "real" stdout
sys.stdout = os.fdopen(1, 'w', 0) # Tell python to use "real" stdout again

# Read and close tmp
tmp.flush()
tmp.seek(0, os.SEEK_SET)
sil.append(tmp.read())

发生错误,输出如下所示:

1.1
1.2
/bin/sh: line 0: echo: write error: Bad file descriptor
print

而sil读:['0.3\n', ''].

换句话说:第二个Func("1.3", True)是无法写入临时文件.

问题

1. 首先,我想知道为什么我的脚本不能像我希望它一样工作.意思是,为什么只能在脚本的前半部分写入临时文件？
2. 使用dup和我仍然有点困惑dup2.虽然我认为我理解如何将stdout重定向到临时文件中,但我现在完全知道为什么os.dup2(new, 1)要做它正在做的事情.也许答案可以详细说明我的剧本中的所有dup和dup2s正在做什么^^

Answer 1

你得到"错误的文件描述符"的原因是垃圾收集器为你关闭了stdout FD.考虑以下两行:

sys.stdout = os.fdopen(1, 'w', 0)    # from first part of your script
...
sys.stdout = os.fdopen(new, 'w', 0)  # from second part of your script

现在当执行这两个中的第二个时,第一个文件对象的引用计数降为零,垃圾收集器将其销毁.文件对象在被破坏时关闭其关联的fd,并且fd恰好是1 = stdout.所以你需要非常小心你如何销毁用它创建的对象os.fdopen.

这是一个显示问题的小例子.os.fstat仅用作示例函数,当您将其关闭为fd时触发"错误文件描述符"错误.

import os
whatever = os.fdopen(1, 'w', 0)
os.fstat(1)
del whatever
os.fstat(1)

我实际上碰巧有一个上下文管理器,我认为完全(或几乎至少,在我的情况下,我发生需要一个命名的临时文件)你正在寻找什么.您可以看到它重用原始的sys.stdout对象以避免出现问题.

import sys
import tempfile
import os

class captured_stdout:
    def __init__(self):
        self.prevfd = None
        self.prev = None

    def __enter__(self):
        F = tempfile.NamedTemporaryFile()
        self.prevfd = os.dup(sys.stdout.fileno())
        os.dup2(F.fileno(), sys.stdout.fileno())
        self.prev = sys.stdout
        sys.stdout = os.fdopen(self.prevfd, "w")
        return F

    def __exit__(self, exc_type, exc_value, traceback):
        os.dup2(self.prevfd, self.prev.fileno())
        sys.stdout = self.prev

## 
## Example usage
##

## here is a hack to print directly to stdout
import ctypes
libc=ctypes.LibraryLoader(ctypes.CDLL).LoadLibrary("libc.so.6")
def directfdprint(s):
    libc.write(1, s, len(s))


print("I'm printing from python before capture")
directfdprint("I'm printing from libc before captrue\n")

with captured_stdout() as E:
    print("I'm printing from python in capture")
    directfdprint("I'm printing from libc in capture\n")

print("I'm printing from python after capture")
directfdprint("I'm printing from libc after captrue\n")

print("Capture contains: " + repr(file(E.name).read()))